The present invention relates to a new bicycle frame tube coupling structure and, more particularly, to a tight coupling which provides a maximized tightening strength for clamping a movable tube set therein without damaging the movable tube and which protects the coupling structure itself against distortion during use.
In a bicycle, a seat stay is adjustably inserted into the rear fork of the bicycle frame and firmly retained therein by means of the tight coupling of the rear fork, so as to support the seat saddle and adjust its elevation.
FIG. 1 shows a common method to position a core tube in a conventional bicycle. As shown in the drawing, a core tube assembly includes a fixed tube set in a tight coupling for retaining a movable tube therein. The fixed tube has an elongated notch transversely made therein piercing its front end, and the tight coupling is cut off at one side to define an elongated opening which is aimed at the elongated notch of the fixed tube when the fixed tube is fixedly set thereinside through welding process. The tight coupling has one pair of symmetric and recessed lugs oppositely disposed for inserting a locking bolt therethrough. A locknut is arranged on the end of the locking bolt so that the coupling can be tightened, thereby forcing the fixed tube to firmly retain the movable tube. In order to reinforce the tightening effect of the tight coupling on the movable tube via the fixed tube, a pressure plate (as shown in FIG. 2-3) is respectively set in the recess of each of the pair of lugs to reinforce the strength of the lugs and to increase the direct pressure onto the fixed tube. The use of such pressure plates is as illustrated in FIGS. 2-1, 2-2 and 2-3. FIG. 2-1 is a vertical sectional assembly view of the said core tube assembly, and FIG. 2-2 is a cross sectional view. The pressure plate, as shown in FIG. 2-3, has an U-shaped configuration and is set in the recess of the associated lug with both ends stopped against the outer wall of the fixed tube. When the locking bolt is tightened the two opposite lugs close inward and the respective pressure plates are forced to press on the fixed tube, so that the elongated notch is closed inward the movable tube is firmly retained. Thus, the movable tube can be firmly positioned. According to this movable tube positioning method, the two opposite lugs are not reinforced in structure. In practice, the two opposite lugs tend to become distorted after a very short period of use, which deteriorates the strength of the tight coupling. Accordingly, the ability of the tight coupling to position and hold the movable tube deteriorates.
The main object of the present invention is to provide a new bicycle frame tube coupling structure which provides better tightening effect, protects the tight coupling against distortion, and guarantees maximum stability and security in operation.
Another object of the present invention is to provide a new bicycle frame tube coupling structure which is easy and more inexpensive to manufacture.
The above and other objects, features and advantages of the present invention will be fully understood from the following detailed description of the preferred embodiment considered in connection with the annexed drawings as hereunder.